1. Technical Field
This invention relates generally to electrostatography, and more specifically to copiers and printers having transfer apparatus for transferring electrostatically held toner images in registry to a receiver sheet.
2. Background Art
In a typical electrostatographic process for making reproductions, an electrostatic charge pattern having an image-wise configuration corresponding to information to be reproduced, is formed on the surface of a grounded image member. The charge pattern is developed by applying developer material to such pattern to form a transferable image on the image member. The developer material includes for example, thermoplastic pigmented marking particles which are attracted to the charge pattern by electrostatic forces. The transferable image is transferred from the image member to a receiver sheet, and permanently fixed to the sheet to form the reproduction. Transfer is accomplished by electrically charging the receiver sheet to a level sufficient to attract the developer material from the image member to the receiver sheet while the sheet is in contract with the area of the image member carrying the transferable image. Electrical charging of the receiver sheet is commonly effected by contacting the surface of the receiver sheet opposite the image member with an electrically biased transfer drum.
An electrically biased transfer drum is suitable for use in an electrostatographic process where multiple related images are transferred in superimposed relation onto a receiver sheet to form a composite reproduction, such as in making a multi-color reproduction. In such a process the receiver sheet is tacked to the transfer drum so that the sheet is successively returned into registered contact with the related transferable images on the image member. An example of such a transfer drum is shown in U.S. Pat. No. 4,712,906, which issued to R. M. Bothner et al. on Dec. 15, 1987.
Such drums often have a vacuum gripping mechanism for attaching the leading edge of a receiver sheet to the drum. However, toner transfer efficiency is generally poor in the region of each vacuum vent, causing an artifact in the transferred image in that region.
Some efforts have been made to reduce such artifacts in the transferred image. For example, commonly assigned U.S. Pat. No. 4,403,847, which issued to G. L. Chestensen on Sept. 13, 1983, discloses a re-transfer apparatus which includes a resilient, deformable electrically conductive drum cover connected to a source of electrical image transferring potential. The conductive drum cover defines a vacuum vent for tacking a receiver sheet. The vent has a longitudinal axis which, at the surface of conductive member, defines an oblique angle to the surface. During image transfer, the drum cover deforms under pressure to eliminate the surface discontinuity at the vent opening so that an electrical transfer potential is uniformly applied. However, the Chestensen system is mechanically complex, and it is not always desirable to use a resilient drum cover.